A known manner of generating a signature comprises of incorporating biometric features of the sender in the digital signature. Biometric features are person-related features based on physical characteristics. Examples of biometric features are finger or toe-prints, voice, speech, retina, iris, geometry or vein pattern of a hand, movement or behavioural characteristics, voice pattern and so on.
A method and a device for digitally signing an electronic document are known from U.S. Pat. No. 7,024,562-B1. A sample is taken from a set of biometric features of the sender. The sample is digitized and added as (part of a) signature to the electronic document to be sent. The combination of signature and document is then encrypted by a hash function. The encrypted combination is sent together with the original document and the original signature. The recipient retrieves the original document and the digital signature from the encrypted combination and compares them to the sent document and the sent signature. If they are the same, it is assumed that the received document is the correct document intended to be sent, without anything having been changed therein during sending.
WO-01/08352-A1 provides a method and an apparatus for providing a document with a signature on the basis of a biometric feature. The signature comprises a sample of a biometric feature taken and digitized at the moment of sending. The document is provided with a time stamp. The signature is then added to the document. Using a personal key the combination of signature, time stamp and document is encrypted.
A drawback of the above described methods is that the signature is always the same. The signature on the document can therefore be imitated after sending. This means that the signature can be copied and reused. A fraudulent person can use the copied signature to sign and send a new document, drawn up by him or her, in the name of the person to whom the signature belongs.